Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit with a fine pattern on e.g. a silicon substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices is being accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (wavelength: 193 nm) is employed, about 45 nm is presumed to be the limit. Under the circumstances, as an exposure technique for the next generation employing an exposure wavelength shorter than 45 nm, EUV lithography is expected to be prospective, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser. In this specification, EUV light is meant for a light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically for a light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm (from about 13.2 to 13.8 nm).
EUV light is likely to be absorbed by all kinds of substances, and the refractive index of substances at such a wavelength is close to 1, whereby it is not possible to use a conventional dioptric system like photolithography employing visible light or ultraviolet light. Therefore, in EUV lithography, a catoptric system, i.e. a combination of a reflective photomask and a mirror, is employed.
A mask blank is a stacked member before pattering, to be employed for the production of a photomask. In the case of an EUV mask blank, it has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, are formed in this order on a substrate made of e.g. glass.
Usually, a protective layer is formed between the above-described reflective layer and the absorber layer. Such a protective layer is one to be provided for the purpose of protecting the reflective layer, so that the reflective layer will not be damaged by an etching process to be carried out for the purpose of forming a pattern on the absorber layer.
As the reflective layer, it is common to use a multilayer reflective film having a low refractive index layer with a low refractive index to EUV light and a high refractive index layer with a high refractive index to EUV light, alternately stacked to have the light reflectivity improved when its surface is irradiated with EUV light. Specifically as such a multilayer reflective film, there is, for example, a Mo/Si multilayer reflective film having a molybdenum (Mo) layer as a low refractive index layer and a silicon (Si) layer as a high refractive index layer alternately stacked.
For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing chromium (Cr) or tantalum (Ta) as the main component, is used.
Usually, a protective layer is formed between the above-described reflective layer and the absorber layer. Such a protective layer is one to be provided for the purpose of protecting the reflective layer, so that the reflective layer will not be damaged by an etching process to be carried out for the purpose of forming a pattern on the absorber layer. In Patent Document 1, it is proposed to use ruthenium (Ru) as the material for the protective layer. In Patent Document 2, a protective layer is proposed which is made of a ruthenium compound (Ru content: 10 to 95 at %) containing Ru and at least one member selected from Mo, Nb, Zr, Y, B, Ti and La.
Further, as disclosed in Patent Document 3, in a mask blank for EUVL, it has been problematic that in-plane distribution of the peak reflectivity in the EUV wavelength region results at the surface of a multilayer reflective film. When a reflectivity spectrum of light in the EUV wavelength region at the surface of the multilayer reflective film is measured, the value of reflectivity varies depending upon the wavelength for measurement, and has a local maximum value i.e. the peak reflectivity. If in-plane distribution of the peak reflectivity of light in the EUV wavelength region at the surface of the multilayer reflective film (i.e. such a state that the peak reflectivity varies depending upon the locations on the multilayer reflective film) results, at the time when EUVL is carried out by using a mask for EUVL prepared from such a mask blank for EUVL, in-plane distribution of the EUV exposure amount applied to the resist on a wafer will result. This causes fluctuations in the dimension of a pattern in the exposure field and thus becomes a factor to impair high precision patterning.
In Patent Document 3, the required value relating to the in-plane uniformity of the peak reflectivity of light in the EUV wavelength region at the surface of the multilayer reflective film is set to be within ±0.25%. Further, in a case where a protective layer is formed on the multilayer reflective film, the required value relating to the in-plane uniformity of the peak reflectivity of light in the EUV wavelength region at the surface of the protective layer is set to be within ±0.25%.
Therefore, with respect to the in-plane uniformity of the peak reflectivity of light in the EUV wavelength region at the multilayer reflective film surface or at the protective layer surface, its range (the difference between the maximum value and the minimum value of the peak reflectivity) is required to be within 0.5%.
Further, as disclosed in Patent Document 3, in a mask blank for EUVL, it is also problematic that in-plane distribution of the center wavelength of reflected light, specifically, in-plane distribution of the center wavelength of reflected light in the EUV wavelength region at the multilayer reflective film surface, results. Here, the center wavelength of reflected light in the EUV wavelength region is, when the wavelengths corresponding to FWHM (full width of half maximum) of the peak reflectivity in the reflectivity spectrum in the EUV wavelength region are represented by λ1 and λ2, a wavelength that becomes the center value of these wavelengths ((λ1+λ2)/2).
In Patent Document 3, the required value relating to the in-plane uniformity of the center wavelength of reflected light in the EUV wavelength region at the surface of the multilayer reflective film is set to be within ±0.03 nm. Further, in a case where a protective layer is formed on the multilayer reflective film, the required value relating to the in-plane uniformity of the center wavelength at the surface of the protective layer is set to be within ±0.03 nm.
Therefore, with respect to the in-plane uniformity of the center wavelength of reflected light in the EUV wavelength region at the multilayer reflective film surface or at the protective layer surface, its range (the difference between the maximum value and the minimum value of the center wavelength) is required to be within 0.06 nm.
One of causes for the above in-plane distribution of reflected light at the surface of the multilayer reflective film i.e. the in-plane distribution of the peak reflectivity of light in the EUV wavelength region at the surface, and the in-plane distribution of the center wavelength of reflected light in the EUV wavelength region at the surface, is in-plane distribution of the thicknesses of the respective layers constituting the multilayer reflective film i.e. the thicknesses of low refractive index layers and high refractive index layers (Patent Document 3). Further, as disclosed in Patent Document 4, fluctuation in thickness (i.e. in-plane distribution of thickness) of a capping layer (i.e. a protective layer) to be formed on the multilayer reflective film will also be a cause for fluctuation of reflected light in the EUV wavelength region (i.e. in-plane distribution of the peak reflectivity of light in the EUV wavelength region, or in-plane distribution of the center wavelength of reflected light in the EUV wavelength region).
Therefore, at the time of film formation for the respective layers constituting the multilayer reflective film, or at the time of film formation for the protective layer, it is required to form a film uniformly in order not to bring about in-plane distribution in the film thickness.
Patent Document 5 discloses a method of forming a multilayer reflective film on a substrate, and a method of forming a capping layer on the multilayer reflective film, by means of an ion beam sputtering method. In such methods, while rotating the substrate about its center axis at the center, ion beam sputtering is carried out by maintaining the angle between the normal line to the substrate and the sputtered particles incident on the substrate to be a certain specific angle. Therefore, in the methods disclosed in Patent Document 5, as shown in FIG. 1 of the same Document, the sputtered particles will enter from an oblique direction to the normal line to the substrate. The methods are a method of forming a multilayer reflective film on a substrate having concave defects on its surface, and a method of forming a capping layer on the multilayer reflective film. However, also in a case where no concave defects are present on the substrate surface, it is desired to carry out the sputtering method under such a condition that while rotating the substrate about its center axis at the center, the sputtered particles will enter from an oblique direction to the normal line to the substrate, in order to form a film uniformly so as not to bring about in-plane distribution in the thickness of the multilayer reflective film or the capping layer to be formed on the multilayer reflective film.